This invention relates to integrated circuit manufacture, and more particularly to a method for forming self-aligned silicided regions on MOS devices.
In the manufacture of MOS integrated circuits, a commonly-used practice is to form silicide on source/drain regions and on polysilicon gates. This practice has become increasing important for very high density devices where the feature size is reduced to a fraction of a micrometer. The purpose of the silicide is to provide good ohmic contacts, reduce the sheet resistivity of source/drain regions and polysilicon interconnections, increase the effective contact area, and provide an etch stop.
One of the preferred materials useful in forming silicided source/drain and polysilicon gate regions in MOS processes is cobalt. The advantages of using cobalt instead of alternative materials such as titanium, platinum, or palladium are that cobalt silicide provides low resistivity, allows shallow junctions, allows lower-temperature processing, has a reduced tendency for forming diode-like interfaces, and etchants for cobalt silicide are stable and can be stored in premixed form indefinitely.
The use of cobalt as a self-aligned silicide material has presented problems in manufacture of high-density integrated circuit devices, due to the tendency to form unwanted cobalt silicide in areas extending onto the insulating sidewall spacers used to self-align the silicided regions. The sidewall spacer is needed to isolate the polysilicon gate from the active areas, and overgrowth of silicide onto these areas can render the device inoperative due to a short. The problem of overgrowth increases as the density increases, because the distance is decreased for which overgrowth causes a catastrophic failure.
Various techniques would reduce the tendency for overgrowth of cobalt silicide in MOS processes. For example, the thickness of the cobalt coating could be decreased, but this results in a reduction in the quality of the cobalt silicide coverage in the areas where it is wanted, i.e., on the source/drain regions. Likewise, the anneal temperature could be reduced, but again this reduces the quality of cobalt silicide in needed areas at the same time as in unwanted areas. Oxygen can be introduced into the chamber during the anneal operation, which decreases overgrowth, but again reduces the quality of silicide growing on the needed areas.
An explanation of the advantages of using cobalt silicide instead of titanium silicide is described by Broadbent et al, "Application of Self-Aligned CoSi.sub.2 Interconnection in Submicrometer CMOS Transistors," IEEE Trans. on Electron Devices, November 1989, pp. 2240-46. A gate leakage problem in devices having cobalt silicide is reported by Swartz et al in "Gate Isolation after Cobalt Silicide Processing," Journal of Electronic Materials, Vol. 19, No. 2, 1990, p. 171; the gate leakage is said to correlate with the presence of an etch-resistant cobalt residue on sidewall spacer surfaces, and it was noted that an oxidation treatment reduced the leakage.